1. Field of the Invention
The present invention relates generally to the compression and liquefaction of gases, and more particularly to the partial liquefaction of a gas, such as natural gas, on a small scale by utilizing a combined refrigerant and expansion process.
2. State of the Art
Natural gas is a known alternative to combustion fuels such as gasoline and diesel. Much effort has gone into the development of natural gas as an alternative combustion fuel in order to combat various drawbacks of gasoline and diesel including production costs and the subsequent emissions created by the use thereof. As is known in the art, natural gas is a cleaner burning fuel than other combustion fuels. Additionally, natural gas is considered to be safer than gasoline or diesel as natural gas will rise in the air and dissipate, rather than settling.
To be used as an alternative combustion fuel, natural gas (also termed xe2x80x9cfeed gasxe2x80x9d herein) is conventionally converted into compressed natural gas (CNG) or liquified (or liquid) natural gas (LNG) for purposes of storing and transporting the fuel prior to its use. Conventionally, two of the known basic cycles for the liquefaction of natural gases are referred to as the xe2x80x9ccascade cyclexe2x80x9d and the xe2x80x9cexpansion cycle.xe2x80x9d
Briefly, the cascade cycle consists of a series of heat exchanges with the feed gas, each exchange being at successively lower temperatures until the desired liquefaction is accomplished. The levels of refrigeration are obtained with different refrigerants or with the same refrigerant at different evaporating pressures. The cascade cycle is considered to be very efficient at producing LNG as operating costs are relatively low. However, the efficiency in operation is often seen to be offset by the relatively high investment costs associated with the expensive heat exchange and the compression equipment associated with the refrigerant system. Additionally, a liquefaction plant incorporating such a system may be impractical where physical space is limited, as the physical components used in cascading systems are relatively large.
In an expansion cycle, gas is conventionally compressed to a selected pressure, cooled, then allowed to expand through an expansion turbine, thereby producing work as well as reducing the temperature of the feed gas. The low temperature feed gas is then heat exchanged to effect liquefaction of the feed gas. Conventionally, such a cycle has been seen as being impracticable in the liquefaction of natural gas since there is no provision for handling some of the components present in natural gas which freeze at the temperatures encountered in the heat exchangers, for example, water and carbon dioxide.
Additionally, to make the operation of conventional systems cost effective, such systems are conventionally built on a large scale to handle large volumes of natural gas. As a result, fewer facilities are built making it more difficult to provide the raw gas to the liquefaction plant or facility as well as making distribution of the liquefied product an issue. Another major problem with large scale facilities is the capital and operating expenses associated therewith. For example, a conventional large scale liquefaction plant, i.e., producing on the order of 70,000 gallons of LNG per day, may cost $2 million to $15 million, or more, in capital expenses. Also, such a plant may require thousands of horsepower to drive the compressors associated with the refrigerant cycles, making operation of the plants expensive.
An additional problem with large facilities is the cost associated with storing large amounts of fuel in anticipation of future use and/or transportation. Not only is there a cost associated with building large storage facilities, but there is also an efficiency issue related therewith as stored LNG will tend to warm and vaporize over time creating a loss of the LNG fuel product. Further, safety may become an issue when larger amounts of LNG fuel product are stored.
In confronting the foregoing issues, various systems have been devised which attempt to produce LNG or CNG from feed gas on a smaller scale, in an effort to eliminate long term storage issues and to reduce the capital and operating expenses associated with the liquefaction and/or compression of natural gas. However, such systems and techniques have all suffered from one or more drawbacks.
U.S. Pat. No. 5,505,232 to Barclay, issued Apr. 9, 1996 is directed to a system for producing LNG and/or CNG. The disclosed system is stated to operate on a small scale producing approximately 1,000 gallons a day of liquefied or compressed fuel product. However, the liquefaction portion of the system itself requires the flow of a xe2x80x9ccleanxe2x80x9d or xe2x80x9cpurifiedxe2x80x9d gas, meaning that various constituents in the gas such as carbon dioxide, water, or heavy hydrocarbons must be removed before the actual liquefaction process can begin.
Similarly, U.S. Pat. Nos. 6,085,546 and 6,085,547 both issued Jul. 11, 2000 to Johnston, describe methods and systems of producing LNG. The Johnston patents are both directed to small scale production of LNG, but again, both require xe2x80x9cprepurificationxe2x80x9d of the gas in order to implement the actual liquefaction cycle. The need to provide xe2x80x9ccleanxe2x80x9d or xe2x80x9cprepurifiedxe2x80x9d gas to the liquefaction cycle is based on the fact that certain gas components might freeze and plug the system during the liquefaction process because of their relatively higher freezing points as compared to methane which makes up the larger portion of natural gas.
Since many sources of natural gas, such as residential or industrial service gas, are considered to be relatively xe2x80x9cdirty,xe2x80x9d the requirement of providing xe2x80x9ccleanxe2x80x9d or xe2x80x9cprepurifiedxe2x80x9d gas is actually a requirement of implementing expensive and often complex filtration and purification systems prior to the liquefaction process. This requirement simply adds expense and complexity to the construction and operation of such liquefaction plants or facilities.
In view of the shortcomings in the art, it would be advantageous to provide a process, and a plant for carrying out such a process, of efficiently producing liquefied natural gas on a small scale. More particularly, it would be advantageous to provide a system for producing liquefied natural gas from a source of relatively xe2x80x9cdirtyxe2x80x9d or xe2x80x9cunpurifiedxe2x80x9d natural gas without the need for xe2x80x9cprepurification.xe2x80x9d Such a system or process may include various clean-up cycles which are integrated with the liquefaction cycle for purposes of efficiency.
It would be additionally advantageous to provide a plant for the liquefaction of natural gas which is relatively inexpensive to build and operate, and which desirably requires little or no operator oversight.
It would be additionally advantageous to provide such a plant which is easily transportable and which may be located and operated at existing sources of natural gas which are within or near populated communities, thus providing easy access for consumers of LNG fuel.
In accordance with one aspect of the invention, a method is provided for removing carbon dioxide from a mass of natural gas. The method includes cooling at least a portion of the mass of natural gas to form a slurry which comprises at least liquid natural gas and solid carbon dioxide. The slurry is flowed into a hydrocyclone and a thickened slush is formed therein. The thickened slush comprises the solid carbon dioxide and a portion of the liquid natural gas. The thickened slush is discharged through an underflow of the hydrocyclone while the remaining portion of liquid natural gas is flowed through an overflow of the hydrocyclone.
Cooling the portion of the mass of natural gas may be accomplished by expanding the gas, such as through a Joule-Thomson valve. Cooling the portion of the mass of natural gas may also include flowing the gas through a heat exchanger.
The method may also include passing the liquid natural gas through an additional carbon dioxide filter after it exits the overflow of the hydrocyclone.
In accordance with another aspect of the invention, a system is provided for removing carbon dioxide from a mass of natural gas. The system includes a compressor configured to produce a compressed stream of natural gas from at least a portion of the mass of natural gas. At least one heat exchanger receives and cools the compressed stream of natural gas. An expansion valve, or other gas expander, is configured to expand the cooled, compressed stream and form a slurry therefrom, the slurry comprising liquid natural gas and solid carbon dioxide. A hydrocyclone is configured to receive the slurry and separate the slurry into a first portion of liquid natural gas and a thickened slush comprising the solid carbon dioxide and a second portion of the liquid natural gas.
The system may further include additional heat exchangers and gas expanders. Additionally, carbon dioxide filters may be configured to receive the first portion of liquid natural gas for removal of any remaining solid carbon dioxide.
In accordance with another aspect of the invention, a liquefaction plant is provided. The plant includes plant inlet configured to be coupled with a source of natural gas, which may be unpurified natural gas. A turbo expander is configured to receive a first stream of the natural gas drawn through the plant inlet and to produce an expanded cooling stream therefrom. A compressor is mechanically coupled to the turbo expander and configured to receive a second stream of the natural gas drawn through the plant inlet and to produce a compressed process stream therefrom. A first heat exchanger is configured to receive the compressed process stream and the expanded cooling stream in a countercurrent flow arrangement to cool to the compressed process stream. A first plant outlet is configured to be coupled with the source of unpurified gas such that the expanded cooling stream is discharged through the first plant outlet subsequent to passing through the heat exchanger. A first expansion valve is configured to receive and expand a first portion of the cooled compressed process stream and form an additional cooling stream, the additional cooling stream being combined with the expanded cooling stream prior to the expanded cooling stream entering the first heat exchanger. A second expansion valve is configured to receive and expand a second portion of the cooled compressed process stream to form a gas-solid-liquid mixture therefrom. A first gas-liquid separator is configured to receive the gas-solid-liquid mixture. A second plant outlet is configured to be coupled with a storage vessel, the first gas-liquid separator being configured to deliver a liquid contained therein to the second plant outlet.
In accordance with another aspect of the invention, a method of producing liquid natural gas is provided. The method includes providing a source of unpurified natural gas. A portion of the natural gas is flowed from the source and divided into a process stream and a first cooling stream. The first cooling stream is flowed through a turbo expander where work is produced to power a compressor. The process stream is flowed through the compressor and is subsequently cooled by the expanded cooling stream. The cooled, compressed process stream is divided into a product stream and a second cooling stream. The second cooling stream is expanded and combined with the first expanded cooling stream. The product stream is expanded to form a mixture comprising liquid, vapor and solid. The liquid and solid is separated from the vapor, and at least a portion of the liquid is subsequently separated from the liquid-solid mixture.